Английская Википедия:Chymosin

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Шаблон:Cs1 config Шаблон:Short description Шаблон:Distinguish Шаблон:Infobox enzyme Chymosin Шаблон:IPAc-en or rennin Шаблон:IPAc-en is a protease found in rennet. It is an aspartic endopeptidase belonging to MEROPS A1 family. It is produced by newborn ruminant animals in the lining of the abomasum to curdle the milk they ingest, allowing a longer residence in the bowels and better absorption. It is widely used in the production of cheese.

Historically, chymosin was obtained by extracting it from the stomachs of slaughtered calves. Today, most commercial chymosin used in cheese production is produced recombinantly in E. coli, Aspergillus niger var awamori, and K. lactis.

Occurrence

Chymosin is found in a wide range of tetrapods,[1] although it is best known to be produced by ruminant animals in the lining of the abomasum. Chymosin is produced by gastric chief cells in newborn mammals[2] to curdle the milk they ingest, allowing a longer residence in the bowels and better absorption. Non-ruminant species that produce chymosin include pigs, cats, seals,[3] and chicks.[1]

One study reported finding a chymosin-like enzyme in some human infants,[4] but others have failed to replicate this finding.[5] Humans have a pseudogene for chymosin that does not generate a protein, found on chromosome 1.[3][6] Humans have other proteins to digest milk, such as pepsin and lipase.[7]Шаблон:Rp

In addition to the primate lineage leading up to humans, some other mammals have also lost the chymosin gene.[1]

Enzymatic reaction

Chymosin is used to bring about the extensive precipitation and curd formation in cheese-making. The native substrate of chymosin is K-casein which is specifically cleaved at the peptide bond between amino acid residues 105 and 106, phenylalanine and methionine.[8] The resultant product is calcium phosphocaseinate.Шаблон:Citation needed When the specific linkage between the hydrophobic (para-casein) and hydrophilic (acidic glycopeptide) groups of casein is broken, the hydrophobic groups unite and form a 3D network that traps the aqueous phase of the milk.

Charge interactions between histidines on the kappa-casein and glutamates and aspartates of chymosin initiate enzyme binding to the substrate.[8] When chymosin is not binding substrate, a beta-hairpin, sometimes referred to as "the flap," can hydrogen bond with the active site, therefore covering it and not allowing further binding of substrate.[9]

Examples

Listed below are the ruminant Cym gene and corresponding human pseudogene:

Шаблон:Infobox nonhuman protein Шаблон:Infobox protein

Recombinant chymosin

Because of the imperfections and scarcity of microbial and animal rennets, producers sought replacements. With the development of genetic engineering, it became possible to extract rennet-producing genes from animal stomach and insert them into certain bacteria, fungi or yeasts to make them produce chymosin during fermentation.[10][11] The genetically modified microorganism is killed after fermentation and chymosin is isolated from the fermentation broth, so that the fermentation-produced chymosin (FPC) used by cheese producers does not contain any GM component or ingredient.[12] FPC contains the identical chymosin as the animal source, but produced in a more efficient way. FPC products have been on the market since 1990 and are considered the ideal milk-clotting enzyme.[13]

FPC was the first artificially produced enzyme to be registered and allowed by the US Food and Drug Administration. In 1999, about 60% of US hard cheese was made with FPC[14] and it has up to 80% of the global market share for rennet.[15]

By 2008, approximately 80% to 90% of commercially made cheeses in the US and Britain were made using FPC.[12] The most widely used fermentation-produced chymosin is produced either using the fungus Aspergillus niger or using Kluyveromyces lactis.

FPC contains only chymosin B,[16] achieving a higher degree of purity compared with animal rennet. FPC can deliver several benefits to the cheese producer compared with animal or microbial rennet, such as higher production yield, better curd texture and reduced bitterness.[13]

References

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Further reading

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External links

  • The MEROPS online database for peptidases and their inhibitors: A01.006

Шаблон:Aspartic acid proteases Шаблон:Enzymes Шаблон:Portal bar

  1. Перейти обратно: 1,0 1,1 1,2 Шаблон:Cite journal
  2. Шаблон:Cite journal
  3. Перейти обратно: 3,0 3,1 Staff, Online Mendelian Inheritance in Man (OMIM) Database. Last updated February 21, 1997 Chymosin pseudogene; CYMP prochymosin, included, in the OMIM
  4. Шаблон:Cite journal
  5. Шаблон:Cite book
  6. Шаблон:Cite book
  7. Шаблон:Cite book
  8. Перейти обратно: 8,0 8,1 Шаблон:Cite book
  9. Ошибка цитирования Неверный тег <ref>; для сносок pmid9862200 не указан текст
  10. Шаблон:Cite journal
  11. Шаблон:Cite journal
  12. Перейти обратно: 12,0 12,1 Шаблон:Cite web
  13. Перейти обратно: 13,0 13,1 Шаблон:Cite book
  14. Шаблон:Cite web
  15. Шаблон:Cite journal
  16. Bovine chymosins A and B differ by one amino acid residue. This is probably an alleic variant, according to Uniprot:P00794. The two isoforms have identical catalytic activity, so any improvement in the product is due to the elimination of other impurities.